This invention generally relates to cardiac pacemakers and more specifically relates to cardiac pacemakers of the type which measure the metabolic demand for oxygenated blood and vary the rate of the pacemaker in accordance therewith.
In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become widely available. Perhaps the most popularly employed method for measuring the need for oxygenated blood is to measure the physical activity of the patient by means of a piezoelectric transducer. Such a pacemaker is disclosed in U.S. Pat. No. 4,485,813 issued to Anderson et al. Alternatively, oxygen saturation may be measured directly as disclosed in U.S. Pat. No. 4,467,807 issued to Bornzin, U.S. Pat. No. 4,807,629 issued to Baudino et al and in U.S. Pat. No. 4,750,495 issued to Brumwell et al. Other parameters employed to measure the metabolic demand for oxygenated blood include right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, respiration, rate, minute ventilation, and various pre and post systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart.
In most cases, the pacemaker includes a sensor which produces an output which varies between a maximum level and a minimum level and provides for a minimum and a maximum pacing rate. In most cases, the rate varies as a linear or monotonic function of the sensor output with the pacing rate being equal to a pre-selected base rate plus an increment which is a function of the measured sensor output (pacing rate=lower rate+f (sensor output). Some temperature sensing pacemakers have employed more complex functions to take the initial dip in temperature due to the onset of exercise into account. In some cases, the function f has a selectable slope (change of pacing rate/change of sensor output) adjustable by means of an external programmer in conjunction with selectable lower and upper pacing rates. While this provides a useful and workable system, it has the disadvantage that the relation between the programned parameters and the behavior of the pacemaker is complex and often not readily apprehended.
For example, in most cases the increment to pacing rate as a function of sensor output (slope) is programmable independent of the selected upper and lower rates. Where there is a wide range of rates between the selected upper and lower rates, the selected slope f may not provide for sufficient incrementation to the base pacing rate at maximum sensor output to actually allow the pacemaker to reach the programmed upper rate. This defeats the physician's intent in selecting the programmed upper rate. Moreover, there are typically only a finite number of selectable slopes for the function relating pacing rate to sensor output so that the wider the rate between the selected upper and lower rates, the fewer available slopes will actually allow the pacemaker to vary between the selected lower and upper rates. This reduces the adjustability of the pacemaker substantially and decreases the physician's ability to fine tune the pacemaker to the patient's physical condition.